In fluid drilling apparatus of the type described in PCT/AU02/01550, it is known to use a gauging ring sized so that the outer diameter of the ring approximates the desired size of the bore hole to be drilled, to control the forward movement of the fluid drilling head through the rock. In the example given in this PCT specification, the gauging ring is spaced from the rotatable nozzle assembly of the drilling head so that chips of rock cut or burst loose by the action of the fluid jets in the rotatable drilling head can pass between the gauging ring and the body of the drilling head to be flushed rearwardly with the return drilling fluid from the pilot jets and reaming jets.
It has been found important to maintain the velocity of the drilling fluid flowing back through the bore hole to avoid the formation or build up of a bed of cuttings in the bore hole which significantly increases the friction between the hose and the walls of a lateral bore hole.
Compared to conventional drilling systems, the thrust forces involved in fluid jet drilling are very low. The drilling head is typically propelled forward by retrojets of drilling fluid from the drilling head with a relatively low thrust force. The hose based system precludes the application of significant “push” which could otherwise be provided by coiled tubing used in other forms of drilling head, as any push of this nature simply helically buckles the hose in the bore hole creating added friction and resistance to forward movement of the drilling head.
It has been demonstrated that the friction between the hose and the lateral bore hole increases many fold when the hose is partially submerged in cuttings being flushed back down the bore hole from the drilling head. It is most desirable for the hose to be in a clean bore hole as the high friction in the hole negates the forward thrust very quickly and restricts the length to which a bore hole can be drilled using a fluid drilling head.
In order to keep the velocity of the fluid flowing in a lateral bore hole high enough to entrain the cuttings and prevent formation or build up of a bed of cuttings in the lateral hole, the fluid velocity must be kept above a critical level in order to entrain the chips or cuttings.
It has been found that fluid drilling heads with gauging rings of the type described in international patent specification PCT/AU02/01550, while useful in preventing the drilling head becoming jammed in the hole through excessively rapid movement, can cause the forward progress of the drill to slow down or stall while the pilot jets cut their way through patches of hard rock or deal with other obstructions. In this situation, the reaming jets continue to enlarge the bore hole diameter and rapidly cut a larger diameter hole than the gauging ring. This large diameter hole results in a slowing of the velocity of the return drilling fluid which in turn leads to a build up of the cuttings bed in the base of the lateral bore hole and increased frictional drag on the hose. Once the friction drag on the hose increases beyond the base level, the tool will not feed at the optimal rate under any circumstance and will proceed drilling a large bore hole. This usually is associated with severe doglegs both horizontally and vertically and the subsequent termination of the lateral bore hole.